Microsoft’s commitment Thursday to open up its APIs and communications protocols and reach out to open source developers polarized opinions much like Microsoft itself has historically polarized the industry.

The European Union expressed outright skepticism over Microsoft‘s four-pronged plan for data portability, support for industry standards, interoperability and openness by publishing APIs and protocols for its client and server operating systems and major enterprise platforms.

Microsoft CEO Steve Ballmer himself said the plan was in part designed to meet obligations laid out in the September 2007 judgment of the EU’s European Court of First Instance (CFI).

Others characterized the announcement as more of the same rhetoric from Microsoft, while some saw it as a bold move to sway next week’s International Standards Organization (ISO) meeting concerning standardization for Microsoft’s Office OpenXML (OOXML) format. Microsoft faces an uphill battle for ISO certification after losing in a first-round vote last year.

But others, including some open source developers and Linux experts, saw promise in Microsoft’s actions even while professing to wait for more details to confirm their optimism.

Still others say while Microsoft is pointing in the right direction, the company’s actions aren’t completely altruistic, and that it still seeks dominance albeit now with an altered fighting stance.

Whatever the reaction, it is clear that Microsoft’s move to loosen its grip on proprietary protocols and APIs represents acknowledgement that Google, open source and other forces are exerting tremendous pressure on traditional software vendors in a world of distributed computing and open Web-based applications.

“The world of software development has been marching in a steady direction toward being open and transparent,” said Jim Zemlin, executive director of the Linux Foundation.

“This announcement by Microsoft seems to indicate they want to participate in that march. Even if some of the announced details still seem less than ideal for open source developers, at least it’s a first step. We look forward to seeing more details on their plans and hope the execution of these plans will bear out these claims.”

Microsoft’s first step included publication on its Microsoft Developer Network (MSDN) Web site of 30,000 pages documenting all the APIs and communications protocols that Microsoft products use to connect to Windows Server 2008 and Windows Vista (including the .Net Framework).

Microsoft will now offer those free to anyone with access to the Web. Previously, they were available only under a trade secret license. The company said developers looking for patent-infringement protection for implementations of Microsoft’s APIs and protocols would be offered licenses at low royalty rates and under reasonable terms.

In the coming months, Microsoft will begin documenting API and communications protocols for SQL Server 2008, Office 2007, Exchange Server 2007 and Office SharePoint Server 2007. Microsoft plans to finish the project in June and said it will continue to reveal APIs and protocols in subsequent versions of those products.

Microsoft also said it will design new APIs for Word, Excel and PowerPoint to allow developers to plug in additional document formats and enable users to set those formats as their defaults.

In addition, Microsoft will document how it supports various standards and the extensions it makes to those standards. And finally, it will launch something called the Open Source Interoperability Initiative that will consist of a set of labs, plug fests and technical content to promote more interoperability between Microsoft software and open source software.

While Microsoft’s API and protocol openness is unprecedented, Goldman Sachs wrote in its analysis that “code sharing was always available for partners that legitimately wanted to build products on Microsoft’s platform.”

The financial firm said for open source companies, “the move will have some positive ramifications for performance when working with Microsoft products.”

Some open source vendors reacted with guarded optimism.

“Customers are tired of proprietary software that offers a one-way road to vendor lock-in,” said Matt Asay, vice president of business development for open source content management vendor Alfresco Software.

“We therefore applaud Microsoft’s efforts to open its APIs and protocols to enterprises and third-party software vendors alike, making it easier for companies like Alfresco to interoperate with Microsoft products.”

Asay said the company would test Microsoft’s data-portability pledge by integrating Alfresco and Microsoft’s SharePoint.

Other vendors wanted Microsoft to put more wood behind its swing at interoperability.

Michael Cunningham, executive vice president and general counsel for Red Hat, chided Microsoft for making similar announcements that always seem to be “strategically timed.”

“Red Hat regards this most recent announcement with a healthy dose of skepticism,” he wrote on his blog.

He called on Microsoft to make three commitments: embrace the Open Document Format and cease pushing OOXML; extend its Open Specification Promise (which offers some Web services protocols royalty free) to all the APIs and protocols it plans to document; and waive the stipulation that open source implementations can be for noncommercial products only.

Microsoft drew a clear distinction between noncommercial and commercial use of technology for which it holds patents.

Brad Smith, senior vice president and general counsel for legal and corporate affairs, said those who want commercial distribution would need a patent license.

“Novell has an agreement that covers those patent rights and other such as Xandros have those patent rights,” he said.

Those two companies last year signed cross-patent licensing deals with Microsoft, a move that came after Microsoft sparked controversy by claiming Linux violates 235 of its patents.

Overall, Microsoft claimed it was a milestone day in its history.

“Open house at Redmond?” 451 Group analysts Matthew Aslett, John Abbott, Nick Selby and Vishwanath Venugopalan wrote in their commentary.

Well, no, but the departure of Bill Gates means some of the holy cows are finally being slaughtered. The rationale, as ever, is to dominate.

Microsoft’s Smith seemed to confirm that belief.

“I believe that today’s step is certainly qualitatively and quantitatively different from any step that we as a company have taken in the past,” said Smith.